This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application COMPOSITION FOR PRODUCTION OF SILICA GLASS USING SOL-GEL PROCESS filed with the Korean Industrial Property Office on Jan. 29, 1999 and there duly assigned Serial No. 2900/1999.
1. Field of the Invention
The present invention relates to a composition for production of silica glass using a sol-gel process, and more particularly to a composition for production of silica glass using a sol-gel process, which exhibits an improved processability allowing fabrication of products having a firm structure while allowing a process thereof using a lathe.
2. Description of the Related Art
Generally, silica glass is transparent and chemically inactive and exhibits superior characteristics in terms of thermal stability and strength while having a relatively low thermal expansion coefficient. By virtue of these characteristics, silica glass is used for optical devices such as optical fibers or optical lenses.
An optical fiber basically has an inner core and an outer cladding having a refractive index different from that of the core to allow total reflection of light in the core. In order to fabricate such an optical fiber, an optical fiber preform is fabricated which has a core rod and an overcladding tube surrounding the core rod. The optical fiber preform is subjected to a thermal treatment, and is then drawn into an optical fiber.
The fabrication of such an optical fiber preform may be achieved using a modified chemical vapor deposition (MCVD) method, a vapor phase axial deposition (VAD) method, or an outside vapor deposition (OVD) method.
In accordance with the MCVD method, an optical fiber preform is fabricated from a high-purity silica glass overcladding tube. This silica glass overcladding tube is fabricated using a sol-gel process.
The sol-gel process is a liquid-phase process capable of exhibiting a high productivity and achieving a free adjustment for the composition of products. This sol-gel process is also highly economical because it is generally carried out at a low temperature. Since a high-purity material is used as the starting material for the sol-gel process, this sol-gel process is very useful for the fabrication of photo masks for semiconductors or the fabrication of high-purity silica glass.
Now, a method for fabricating an overcladding tube made of silica glass using such a sol-gel process will be described in brief.
First, silica particles are dispersed in water, thereby forming a sol. The formed sol is kept, as it is, for a desired period of time, thereby aging the sol. The aged sol is then placed into a mold to induce a gelation thereof. After completion of the gelation, the resultant gel is separated from the mold, and then dried.
The dried gel is subjected to a primary thermal treatment in order to remove organic substances from the gel. A silica glass overcladding tube is fabricated using the resultant gel while carrying out a reaction for removal of hydroxy groups and a sintering reaction.
The reactivity of the above mentioned sol-gel process depends on factors such as the gelation reaction, the composition, pressure and pH of the sol used, and the solvent used. However, it is difficult to control this reactivity within a desired range in accordance with appropriate adjustments of the above mentioned factors. The formed gel may be highly vulnerable to cracking during the drying thereof. During the sintering process, the gel may also be subjected to a shrinkage and cracking. In order to solve such problems, various methods have been proposed which include a method using a drying control chemical additive (DCCA), a re-dispersion method, a supercritical drying method, and a method using a polymeric binder.
In accordance with the method using a DCCA, the solvent in the gel exhibits a low local volatilization rate difference. Accordingly, a minimized local stress difference in the gel is exhibited during drying the gel. As a result, the gel has an increased hardness, so that it exhibits a reduced formation of cracks.
In accordance with the re-dispersion method, fine dried silica powder, namely, fumed silica, is dispersed in water, thereby forming a sol which is, in turn, subjected to a gelation process. During the gelation process, the sol is formed into an agglomerate due to hydrogen bonding of silica particles. The formed agglomerate is dried, thermally treated, and ground, so that it becomes powder. The resultant powder is re-dispersed in water. The resultant product obtained after the re-dispersion is gelled in a mold, and then sintered to form a product having a desired shape.
However, all the above mentioned methods are not sufficiently effective in suppressing the generation of cracks occurring during drying the gel. Furthermore, they have a problem in that complex processes are used. In order to solve such problems, a method has been proposed which uses, as a binding agent, an acrylic resin produced by a polymerization of a monomer, such as acrylamide or methacrylamide, with a crosslinking agent. In accordance with this method, the cracking phenomenon occurring during a drying process is reduced. However, this method has a problem in that methacrylamide or acrylamide, which is used as a monomer, is undesirable in terms of environmental protection.
It is therefore an object of the present invention to provide an improved composition and process for production of silica glass.
A further object of the present invention is to efficiently suppress crack formation during gel drying in a sol-gel process.
A yet further object of the invention is to provide a composition for production of silica glass which does not contain methacrylamide, acrylamide or other environmentally undesirable monomers.
A still further object of the invention is to provide a gel which has improved shaping processibility.
A still yet further object of the invention is to provide a gel which can be shaped using a lathe.
In accordance with the present invention, these objects are accomplished by providing a composition for production of silica glass using a sol-gel process comprising: silica particles; a dispersing agent; a polymerizable monomer for formation of acrylic resin and a crosslink agent dissolved in distilled water, the polymerizable monomer being n-methylolacrylamide or n-methylolmethacrylamide; and a solvent.
In the method of the present invention, a premix solution is prepared containing a polymerizable monomer and a cross-linking agent in a solvent. The polymerizable monomer of the present invention is n-methylolacrylamide or n-methylolmethacrylamide. Silica particles and a dispersant are added to the premix solution and dispersed to form a sol. A plasticizer may also be added. The mixing ratio between silica particles and the premix solution is preferably in the range of 4:6 to 6:4 by weight.
A polymerization initiation agent is added, and a catalyst or a gelling agent may be added. The sol is placed in a mold for gelling. The resulting gel is removed from the mold, aged, dried and treated to provide a silica glass tube. The resulting gel may also be shaped using a lathe.
In accordance with the present invention, a macromolecule, which is produced to have a three-dimensional network structure by polymerizing a premix solution containing a polymerizable monomer for formation of acrylic resin and a crosslink agent dissolved in distilled water, is used as a binding agent to produce silica glass using a sol-gel process. For the polymerizable monomer, n-methylolacrylamide (CH2=CHCONHCH2OH) or n-methylolmethacrylamide (CH2=C(CH3)CONHCH2OH) is used in accordance with the present invention. Such a macromolecular binding agent is uniformly distributed in the entire portion of a gel. Accordingly, it is possible to reduce or prevent problems involved in a general sol-gel process, that is, a density gradient, while suppressing the generation of cracks in a sol during drying the sol, and achieving a reduction in drying time.
Preferably, the polymerizable monomer is used in a content of 2 to 20 weight % based on the weight of the premix solution. Where the content of the polymerizable monomer is less than 4 weight % based on the weight of the premix solution, it is preferred that a gelling agent be added to the premix solution to adjust the pH of the premix solution to be 9 to 11, thereby promoting the gelation of a sol formed. On the other hand, where the content of the polymerizable monomer is not less than 4 weight % based on the weight of the premix solution, it is possible to produce silica glass having desired physical properties without any additional pH adjustment.
For the crosslink agent,. any crosslink agents typically used in the production of silica glass are usable. Preferably, N,Nxe2x80x2-methylenebisacrylamide is used as the crosslink agent. Preferably, the crosslink agent is used in a content of 0.05 to 1.0 weight % based on the weight of the premix solution.
It is also preferred that the concentration of the solid part in the premix solution including the polymerizable monomer and the crosslink agent be ranged from 2 weight % to 21 weight % based on the weight of the premix solution in order to avoid a residue of carbon after an organic decomposition involved in the sol-gel process. Where the concentration of the solid part in the premix solution is within the above mentioned range, a moist gel produced from the premix solution exhibits softness. In this case, the gelation is efficiently carried out, thereby achieving an improvement in the strength of the moist gel.
Now, a method for producing silica glass from the above mentioned composition using a sol-gel process in accordance with the present invention will be described in detail.
In accordance with the method of the present invention, a premix solution is first prepared by dissolving a polymerizable monomer and a crosslink agent. Silica particles are dispersed in the prepared premix solution along with a dispersing agent, thereby forming a sol. The dispersing agent serves to allow the silica particles to be uniformly dispersed in the premix solution. Preferably, the dispersing agent includes tetramethyl ammonium hydroxide or the like.
If necessary, an additive such as a plasticizer may be added to the sol. For the plasticizer, polyhydric alcohol is preferably used. The polyhydric alcohol may include glycerin, ethylene glycol, or 2-methylpropyne-1,2,3-triol.
Preferably, the mixing weight ratio between the silica particles and the premix solution is within a range of 4:6 to 6:4. Where the premix solution is used in an amount beyond the above mentioned range, there are problems in that a gel formed from the sol may be highly vulnerable to cracking during the drying thereof or that it is difficult for silica particles to be dispersed in the premix solution. After the formation of the sol, bubbles existing in the sol are removed using a vacuum pump. Thereafter, the resultant sol is aged for a desired period of time to stabilize the silica particles therein.
A polymerization initiating agent is then added to the resultant sol. The resultant product is placed into a mold so that it is gelled in the mold. For the polymerization initiating agent, 2 imidazoline-2-yl is preferable. Of course, the polymerization initiating agent is not limited to this material. When a polymerization initiating agent is added to the sol, a gelling agent or a catalyst may be added to the sol, if necessary. For instance, where the content of the polymerizable monomer is less than 4 weight %, a gelling agent is used to adjust the pH of the sol to 9 to 11, thereby promoting the gelation of the sol. In this case, the gelling agent is added in an amount typically used in the fabrication of silica glass. For the catalyst, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylenediamine (TEMED) is preferably used.
The resultant gel is aged at room temperature. The aged moist gel is removed from the mold, and then dried in a constant temperature and humidity device maintained at a temperature of 20 to 50xc2x0 C. and a relative humidity of 70 to 95%.
The dried gel is then treated to form a silica tube. The following conditions are exemplary. The dried gel is heated to a temperature of 300 to 700xc2x0 C. at a rate of approximately 50xc2x0 C. per hour in the atmosphere, and then maintained at the temperature for 2 to 8 hours. Thereafter, the gel is heated again to a temperature of 800 to 1,200xc2x0 C. at a rate of approximately 100xc2x0 C. per hour in a chlorine gas atmosphere, and then maintained at the temperature for 1 to 8 hours. Subsequently, the gel is heated again to a temperature of 1,100 to 1,500xc2x0 C. at a rate of approximately 100xc2x0 C. per hour in a helium gas atmosphere, and then maintained at the temperature for 1 to 8 hours. Thus, a silica glass tube is fabricated.